Vascular smooth muscle-inspired architecture enables soft yet tough self-healing materials for durable capacitive strain-sensor

Catastrophically mechanical failure of soft self-healing materials is unavoidable due to their inherently poor resistance to crack propagation. Here, with a model system, i.e., soft self-healing polyurea, we present a biomimetic strategy of surpassing trade-off between soft self-healing and high fracture toughness, enabling the conversion of soft and weak into soft yet tough self-healing material. Such an achievement is inspired by vascular smooth muscles, where core-shell structured Galinstan micro-droplets are introduced through molecularly interfacial metal-coordinated assembly, resulting in an increased crack-resistant strain and fracture toughness of 12.2 and 34.9 times without sacrificing softness. The obtained fracture toughness is up to 111.16 ± 8.76 kJ/m2, even higher than that of Al and Zn alloys. Moreover, the resultant composite delivers fast self-healing kinetics (1 min) upon local near-infrared irradiation, and possesses ultra-high dielectric constants (~14.57), thus being able to be fabricated into sensitive and self-healing capacitive strain-sensors tolerant towards cracks potentially evolved in service.

, the fabricated SSPUGIT-C-3 control sample was heterogeneous, where the Galinstan was partially escaping from SSPU matrix due to the extrusion combined with hot pressing method. These overflow features were further conformed by optical microscopy image ( Supplementary Fig. 11b). As depicted, vast majority of Galinstan was distributed in SSPU in a messy way, which is totally different with the vascular smooth muscle-inspired structure of SSPUGIT-3 (Fig. 1e).
Meanwhile, the size of the Galinstan droplets in SSPUGIT-3-C is much smaller than that of SSPUGIT-3, indicating that original micron-sized Galinstan droplets in SSPUGIT-3 were broken into more and smaller ones during the extrusion combined with hot pressing process, together with the formation of many rigid gallium oxide particles agglomerated in SSPUGIT-3-C matrix. High content of filler agglomeration would weaken the tensile stress and strain inevitably 8  SSPU, when the temperature increases from 25 to 80 ℃, the intensity of peaks at 1634 cm -1 (H-bonded -C=O motifs in urea) and 1572 cm -1 (H-bonded -N-H motifs in urea) gradually decreased, and the peaks at 1692 cm -1 (free -C=O motifs in urea) and 1519 cm -1 (free -N-H motifs in urea) gradually increased. These spectral features suggested the hydrogen bonding between different urea moieties gradually broken with the increase of temperature. However, as for SSPUGIT, the peak intensity of H-bonded -C=O motifs continued to decrease during the whole heating process, while the peak intensity of the free -C=O motifs is not changed at all. The phenomenon suggested that there were no free -C=O motifs generated in heating, probably due to the formation of robust coordination interaction between -C=O motifs and Ga2O3 shell 9 . Figure 14. UV-vis absorption spectra of SSPU, Galinstan and SSPUGIT-3 dissolved or dispersed in trichloromethane. As shown, the characteristic peak of SSPU dissolved in trichloromethane was centered at 274.81 nm, while the characteristic plasmon resonance for Galinstan particles dispersed in trichloromethane was centered at 273.62 nm. However, these peaks shifted to a higher wavelength of 276.90 nm for SSPUGIT-3, which is attributed to the coordination interaction between -C=O motifs and Ga2O3 shell 10 .

Supplementary Figure 15. (a) Top view and (b) bottom view of photographs and the corresponding
optical microscopy images of the fabricated control composite, in which the SSPU matrix was synthesized by PDMS chains with a high molecular weight (Mn = 5000-7000 g mol -1 ). As depicted in the photograph, the top view of the control composite looked uniform, but its bottom image exhibited obviously inhomogeneous argent particles. Meanwhile, as shown in optical microscopy images, Galinstan droplets were rarely visible in the top view, which settled and accumulated at the bottom of material to form macro-phase separation. Moreover, the size of the Galinstan droplets was heterogeneous, which may be due to the lack of sufficient interfacial interactions between SSPU chains and Galinstan to constrain agglomeration behavior.
These phenomena indicated that the robust interfacial coordination interaction derived from -C=O motifs and gallium oxide shell was indispensable, which was a necessary prerequisite of successfully forming vascular smooth muscle-inspired structure to achieve soft yet tough self-healing materials. Galinstan content to 400 wt%. According to Rivlin-Thomas single-notch method 11 , fracture energy (Γ) was proportional to strain energy density (Wc), i.e., Γ ∝ Wc. However, the Wc value of SSPUGIT-4 was smaller than that of SSPUGIT-3 composite. By comparison, both SSPUGIT-3 and SSPUGIT-4 were highly crackresistant that the notched sample possessed almost the same tensile strain as the intact sample. However, the tensile stress and tensile strain of the intact samples of SSPUGIT-4 is smaller than that of SSPUGIT-3 (black lines in Supplementary Fig. 16), which directly resulted in the decrease of Wc for SSPUGIT-4 composite. As shown in Supplementary Fig. 16, the tensile stress and tensile strain of the intact sample of SSPUGIT composite gradually decreased as Galinstan content increases. These reductions were in line with the classical linear elastic fracture mechanics (LEFM) theory that composites with much more filler droplets would break at lower stress (and hence lower strain) than composites with less droplets 12 . From another perspective, the droplets within composite matrix acted like the flaw. The reduced tensile strain also agreed with the theory than tensile strain markedly decreases when the flaw number increases 13 . For these reasons, the strain energy density (Wc) of SSPUGIT-4 decreased, and the corresponding fracture energy decreased as well.
Supplementary Figure 17. The initial prats of the stress-strain curve of tensile test of SSPU. As shown, this curve was linearly fitted to achieve the slope, which corresponded to the Young's modulus of SSPU. defined as the energy dissipated during loading and unloading process, which could be calculated by integrating the area of cyclic tensile curves 11 . The damping capacity was defined as the ratio of energy dissipation to incoming energy 14 . As shown, SSPUGIT-3 dissipated more energy than SSPU at each strain (200%, 400%), which was attributed to the breakage of outer Ga2O3 shell and rupture of the interfacial coordination interactions between Ga2O3 and SSPU.

Supplementary Figure 21. Schematic illustration of deflection, branching and elimination crack for
SSPUGIT composite. As shown, with the increase of tensile strain to ~200% strain, outer Ga2O3 shell of Galinstan particles was first broken and the interfacial coordination interactions between Ga2O3 and SSPU matrix was then destroyed accordingly. Subsequently, the inner liquid cores were released and further elongated into fiber-like ellipsoids as we continued to stretch (>200%).

Supplementary Figure 22. Storage modulus, loss modulus and loss factor versus temperature for SSPU.
As shown, there were two distinct relaxation process at -13.21 ℃ and 90.22 ℃, which was assigned to the dynamics of hard domains (α' relaxation) and terminal relaxation of polymer chains (that is the crossover temperature between storage and loss modulus), respectively. According to loss factor curve (green line), the terminal relaxation started at room temperature, and was fully accompanied by an increase in tan δ to high temperature. This phenomenon was also reported by our group 15 , manifesting that polymer chains had a tendency to flaw at room temperature, thus leading to the good room-temperature self-healing kinetics. output power was controlled to make the polymer chains flow rapidly to induce high-accuracy self-healing. It could be seen that the terminal relaxation temperature of SSPUGIT-3 was a little higher than that of SSPU ( Supplementary Fig. 22), which was because the incorporation of Galinstan droplets into SSPU matrix hindered the movement of polymer chains of SSPU to some extent.  Supplementary Fig. 32a, the as-made sheet was first cut into pieces and then rapidly dissolved by trichloromethane in a shear mixer. Such processes resulted in a SSPU and trichloromethane solution with dispersed Galinstan droplets, which was similar to the original composite system. Subsequently, this composite dispersion could be cast into a new sheet via a solvent evaporation method. Satisfactorily, the microstructures of the remolded composites were almost constant even after three cycles (Supplementary Fig.   32b; Fig. 1e). But more than that, the mechanical properties of the recycled composite were also basically maintained at the original levels ( Supplementary Fig. 32c), demonstrating the potential for reusing the composite.  Supplementary Fig. 33a, SSPUGIT-3 composite was dissolved in CHCl3, which was then centrifuged to obtain Galinstan droplets and SSPU-loaded CHCl3 solution. Noting that the centrifuge effluent consisted of micro-and nanodroplets wrapped in layers of gallium oxide. To achieve a macroscopic Galinstan droplets, base (i.e., 0.6 M NaOH) was employed to remove the oxide layer 16 .
As a result, the gathered minuscule droplets were driven to merge into a macroscopic and reusable Galinstan droplets by their high interfacial tension, together with a high recovery efficiency of 84±2% 17 . Intriguingly, the recycled Galinstan droplets could be reused to fabricate new composite, which possessed similar tensile curves as the original composite ( Supplementary Fig. 33b). to 400% and recovering in 2 h. Following an initial loading cycle, the second loading cycle of SSPUGIT-3 was basically identical to the third cycles with small hysteresis at low strains (<60%) (Supplementary Fig.   40a), demonstrating the fast and elastic deformation behavior, which is highly desired for application as capacitive strain-sensors. Indeed, the difference in the initial loading cycle was attributed to the Mullin's effect 21 , which could be observed each time upon the specimen was stretched to a new strain exceeding the previous maximum strain. When the specimen was loaded to a high strain region of 400%, a large hysteresis was observed at the initial loading cycle (Supplementary Fig. 40b), which was not only ascribed to the influence of Mullin's effect, but also due to the breakage of weak hydrogen bonds within dynamic hard domains of SSPU to dissipate strain energy. Notably, the loading and unloading curves of subsequent cycle almost overlapped after being stretched to 400%, suggesting that the hydrogen bonds could completely reform in the relaxation time of 2 h (Supplementary Fig. 40b). Such an excellent recovery ability was also depicted in Supplementary Fig. 40c, in which the elongated SSPUGIT-3 was observed to fully return its original dimension without residual strain after relaxation for 2 h.  Supplementary Fig. 40a), the deformation recovery of SSPUGIT-3 is fast and elastic with small hysteresis at low strain (<60%), making it an ideal candidate for wearable capacitive strain-sensor. Specifically, when this strain-sensor was conformally laminated onto elastic substrates such as finger or wrist, it showed clear, sensitive and stable signals during cyclic human motion detection without any rest (Supplementary Fig. 41).